Tag Archives: runaway universe

Map of CMB temperature fluctuations with slightly colder areas in blue, and hotter areas in red.

The first results from the European Space Agency’s Planck satellite have provided excellent confirmation for the Lambda-CDM (Dark Energy and Cold Dark Matter) model. The results also indicate somewhat more dark matter, and somewhat less dark energy, than previously thought. These are the most sensitive and accurate measurements of fluctuations in the cosmic microwave background (CMB) radiation to date.

Results from Planck’s first 1 year and 3 months of observations were released in March, 2013. The new proportions for mass-energy density in the current universe are:

Ordinary matter 5%

Dark matter 27%

Dark energy 68%

Credit: European Space Agency and Planck Collaboration

The prior best estimate for dark matter primarily from the NASA WMAP satellite observations, was 23%. So the dark matter fraction is higher, and the dark energy fraction correspondingly lower, than WMAP measurements had indicated.

Dark energy still dominates by a very considerable degree, although somewhat less than had been thought prior to the Planck results. This dark energy – Lambda – drives the universe’s expansion to speed up, which is known as the runaway universe. At one time dark matter dominated, but for the last 5 billion years, dark energy has been dominant, and it grows in importance as the universe continues to expand.

The Planck results also added a little bit to the age of the universe, which is measured to be about 13.8 billion years, about 3 times the age of the earth. The CMB radiation itself, was emitted when the universe was only 380,000 years old. It was originally in the infrared and optical portions of the spectrum, but has been massively red-shifted, by around 1500 times, due to the expansion of the universe.

There are many other science results from the Planck Science team in cosmology and astrophysics. These include initial support indicated for relatively simple models of “slow roll” inflation in the extremely early universe. You can find details at the ESA web sites referenced below, and in the large collection of papers from the 47th ESlab Conference link.

Future for our Sun: Ultraviolet image of the planetary nebula NGC 7293 also known as the Helix Nebula. It is the nearest example of what happens to a star, like our own Sun, as it approaches the end of its life when it runs out of fuel, expels gas outward and evolves into a much hotter, smaller and denser white dwarf star. Image Credit: NASA/JPL-Caltech/SSC

In the future, the average density of matter in the universe (both ordinary matter and dark matter) will continue to drop in proportion to the increasing spatial volume as the universe expands ever more rapidly. The dark energy density, however, behaves differently. Dark energy is an irreducible property of even empty space, so as new space is created, the dark energy density remains the same; it is believed to not only take the same value in all portions of space at a given time, but to also have had the same value (per unit volume) for many billions of years.

Since around 5 billion years ago, when the universe was 9 billion years old, the dark energy has dominated over both types of matter (ordinary and dark) and this dominance is only increasing with the universe’s continued expansion. Today it is 73% of the total mass-energy density and it will approach close to 100% in the future. The assumption is made that the cosmological constant or dark energy term that we measure today remains constant into the future. However it cannot be ruled out that it is changing very slowly or might change suddenly at some future date.

In the cosmological constant case, the scale factor for the size of the universe grows exponentially with time. This is known as the de Sitter solution to the equations of general relativity, and it indicates that the expansion of the universe is accelerating into a runaway condition. There is a single parameter, a timescale. Cosmological measurements indicate that the value is such that the size of the universe for each spatial dimension will double and redouble every 11 billion years (the volume will thus grow by 8 times each 11 billion years).

When the universe is 25 billion years old (now it’s 14 billion years old), distant galaxies will be about twice as far away as today (and 4 times fainter). Well before that time we’ll need to evacuate the Earth as the Sun will go into its red giant phase some 5 billion years from now, followed by a white dwarf phase – as shown in the image of the Helix planetary nebula above. When the universe is around 124 billion years old, distant galaxies on average will be 1000 times farther away from us than now. And after 234 billion years they will be an incredible million times farther away than now!

YearRelative DistanceRelative Brightness

14 billion (Now) 1 1

25 billion 2 1/4

124 billion 1000 one-millionth

234 billion 1,000,000 one-trillionth

The distant galaxies that we detect with the Hubble telescope and large Earth-bound telescopes will become invisible since their apparent luminosity will drop as the square of the increasing distance. For example at the time of 124 billion years, they will be 1 million times fainter (1000 squared). At the time of 234 billion years they will be a trillion times fainter (one million squared). Actually it will be worse than this since their light will be redshifted (stretched out by the cosmological expansion) by the same relative distance factor, so light emitted in the visible will be detected in the millimeter radio region when the universe is 100+ billion years old. This is without considering the evolution in their stellar populations, but only their lower mass, fainter stars will survive, further aggravating the situation.

Galaxies themselves are not changing very much in their size or in internal density, rather it is the spacing between galaxies that is on average growing rapidly. Galaxy groups and clusters that are today gravitationally bound will remain bound. Our home, the Milky Way galaxy, and its large neighbor the Andromeda galaxy, will stay together since they are gravitationally bound, and they may very well merge in several billion years due to tidal effects. All of the 40 or so galaxies and dwarf galaxies in our gravitationally bound Local Group may coalesce after 1 trillion years have passed.

Our light cone horizon, which determines which galaxies are even theoretically visible to us, is shrinking in relative terms. Sufficiently distant galaxies are already receding faster than the speed of light from our vantage point and are entirely hidden from us; if the inflationary model is correct as seems to be the case, the universe is immensely larger than what we are able to detect. This is possible and indeed happening because there are no constraints in special relativity or general relativity on the expansion rate of space itself; only the objects within space are constrained to moving at less than the speed of light relative to their local frames of reference.

An intelligent society in the very distant future, possibly our descendants who have moved to a planet in orbit around another star, would observe only one galaxy, namely their own. This would be a larger galaxy formed from the Milky Way and other members of the Local Group. All other galaxies would no longer be visible, first they would become too distant and too faint, and then they would be entirely beyond our light horizon. These descendants or other observers would believe their galaxy to be the only one in the universe, unless they had access to (and a willingness to believe in) very ancient research publications.

We are fortunate to live in this epoch – despite dark matter, dark energy, and dark gravity, the universe is young, and we are immersed in light.

Dark energy was first introduced as a possibility as a result of the formulation of Einstein’s equations of general relativity. When he considered how the universe as a whole would behave under the general relativity description of gravity, he added a term to his equations, known as the cosmological constant. At the time the prevailing view was that the universe was static, and neither expanding nor contracting. The term was intended to balance the self-gravitational energy of the universe, and it thus acts as a repulsive force, rather than an attractive one. His basis for introduction of the cosmological constant was erroneous in two respects. The first problem is that the static solution was unstable, as if balanced on a knife edge. If you nudged it a little bit by increasing the matter density in some region slightly, that region would collapse, or if you lowered the density ever so slightly, that region would expand indefinitely. The second problem is that by 1929 Edwin Hubble had demonstrated the universe is actually expanding at a significant rate overall.

Subsequently, Einstein called the introduction of the cosmological constant his “greatest blunder”. After the realization that we live in an expanding universe, while the possibility of the cosmological constant having a non-zero value was sometimes entertained in cosmological theory, it was mostly ignored (set to zero). Over the next several decades, attention turned to better measuring the expansion rate of the universe and the inventory of matter, both ordinary matter and the dark matter, with the amount of the latter implied by long range gravitational effects seen both within galaxies and between galaxies. Was there enough matter of both types to halt the expansion? It seemed not, rather that there was only about 1/4 of the required density of matter, and that was mostly in the form of dark, not ordinary matter. Matter of either type would slow down the expansion of the universe due to its gravitational effects.

After 1980, the inflationary version of the Big Bang gained acceptance due to its ability to explain the flat topology of the universe and the homogeneity of the cosmic microwave background radiation, the relic light from the Big Bang itself. The inflationary model strongly indicated that the total energy density should be about 4 times greater than seen from the matter components alone. It is the total of energy and matter (the energy content of matter) which determines the universe’s fate, since E = mc^2.

In 1998 the astounding discovery was made that the universe’s expansion rate is accelerating! This was determined by two different teams, each of which were making measurements of distant supernovae (exploding stars). And it was confirmed by measurements of tiny fluctuations in the intensity of the microwave background radiation. The two techniques are consistent, and a third technique based on X-ray emission from clusters of galaxies, as well as a fourth technique based on very large scale measurements of relative galaxy positions, also give results consistent with the previous two techniques. The inflationary predictions are satisfied with dark energy presently three times more dominant than the rest mass energy equivalent from dark matter plus ordinary matter. Further measurements have refined our understanding of the relative strength of dark energy in comparison to dark matter and ordinary matter. The best estimates are that, today, dark energy is 74% of the universe’s total mass-energy balance.

In the cosmological constant formulation, dark energy is constant in time, while the matter density drops as the universe expands, in proportion to the cube of the scale factor. So if we consider the universe in its early days the energy contained in the dark matter would have dominated over dark energy, as the mass density would have been much greater than today. The crossover from matter dominated to dark energy dominated came after the universe was about 9 billion years old, or about 5 billion years ago. This emergence of dark energy as the dominant force, due to its nature as a repulsive property of “empty” space-time, results in an accelerating expansion of the universe, which has been called the “runaway universe”. Our universe is apparently slated to become hugely larger than its current enormous size.

Why is dark energy important then? Since five billion years ago, and on into the indefinite future, it has dominated the mass-energy content of the universe. It drives a re-acceleration of the universe. It inhibits the re-collapse (“Big Crunch”) of our entire universe or even substantial portions of the universe. Thus it naturally extends the life of the entire universe to trillions of years or much more – far beyond what would occur were the universe to be dominated by matter only and with density at the critical value or above. Dark energy thus works to maximize the available time and space for life to develop and to evolve on planets found throughout the universe.